The present invention relates to a field effect transistor (FET), especially for use as a sensor element or in an acceleration sensor, a method for its manufacture, and a sensor element of this type according to the species of the independent claims.
Acceleration sensors, or inertial sensors, are widely known. In this context, in addition to precision conceptual approaches, there are also micromechanical conceptual approaches, in which both discrete components as well as components integrated in semiconductor circuits are conventional.
One example of an acceleration sensor of this type is described in U.S. Pat. No. 5,503,017. In this acceleration sensor, two FETs, opposite each other, are provided within a semiconductor substrate in the area of a recess in this substrate. The FETs have a common gate electrode substantially filling up the recess. Under the influence of an external force acting upon this acceleration sensor, and the gate electrode, the result is a change in the distance between the gate electrode and the two channel regions of the FETs opposite each other, a reduction in the distance between the gate electrode and one of the channel regions corresponding to an increase in the distance regarding the other channel region. In this manner, it is possible in principle to measure one component of this external force.
In addition, in the Japanese Patent Application No. 4-25764, whose content is summarized in U.S. Pat. No. 5,503,017, an acceleration sensor is described which is based on an FET. A substrate is furnished with a drain area and a source area, which are separated from each other by a channel region. Furthermore, in that publication, a process of mounting a substantially self-supporting, flexibly supported gate electrode above the channel region, the gate electrode being separated by a gap from the channel region in the planar substrate below is described. In this context, under the influence of an external force acting perpendicular to the surface of the substrate, the result is a reduction in the width of the gap and therefore a change in the drain current in the FET. In this manner, this component of the external force acting perpendicular to the surface of the substrate can be detected via the measured current.
In addition to the micromechanical conceptual approaches discussed above for acceleration sensors, conventional acceleration sensors are manufactured using electroplating additive technology. In this regard, reference should be made, for example, to German Patent Application No. 196 37 265 or German Patent Application No. 197 19 601.
The method for manufacturing acceleration sensors of this type, i.e., the electroplating additive technology itself, is described in German Patent No. 44 18 163 and in German Patent Application No. 197 32 250.
An FET according to the present invention and a sensor element according to the present invention, having at least one FET of this type, has the advantage that the acceleration to be measured does not first have to be transformed into a motion, and then into a change in capacitance, and finally into a change in voltage. Instead, the motion of the gate electrode employed as the inert mass in the FET is converted directly into an easily measurable change in the drain current of the FET.
Furthermore, the FET according to the present invention has the advantage that the deflection of the gate electrode from the neutral position, acting at least approximately parallel to the surface of the substrate, is at least approximately linear with respect to a component of an impinging external force acting parallel to the surface of the substrate. In this manner, the disadvantage is removed that attaches to the conventional acceleration sensors that are based on a capacitive evaluation principle, namely that a nonlinearity exists in principle between the capacitance signal and the acceleration or deflection signal of the inert mass. Furthermore, the capacitance swings arising in acceleration sensors functioning on the basis of the capacitance evaluation principle are usually very small, so that demands that are difficult to satisfy must be placed upon the unit that transforms change in capacitance into change in voltage as well as upon the design and the bonding technology in acceleration sensors of this type. These demands do not apply to FETs according to the present invention.
Rather, in the case of FETs according to the present invention, the sensor signal advantageously exists directly as a current or voltage signal, and does not need to be transformed as a change in capacitance first into a voltage signal, as is usually the case. In this manner, particularly high sensitivities and resolutions can be obtained in the FET according to the present invention or in the sensor element according to the present invention, because interfering parasitic effects, such as noise, are significantly suppressed.
The FET according to the present invention is significantly simpler in its design and is more reliable and simpler than the conventional concepts with regard to evaluating the measuring signal.
Finally, the FET according to the present invention and the method for manufacturing it according to the present invention have the advantage that the FET, including the gate electrode, can be manufactured in one standard CMOS process, and that for manufacturing the necessary integrated circuits (IC) conventional ICs can be used which have been manufactured previously in a separate, generally conventional standard CMOS process. To this extent, a conventional IC for FETs can be initially used advantageously as the substrate, on which, after the IC has also been manufactured using a generally conventional electroplating additive technology, the substantially self-supporting gate electrodes are additionally mounted in the form of movable, acceleration-sensitive structures, along with corresponding contacts and anchoring points.
In this manner, the FET according to the present invention can advantageously be produced in a very cost-effective manner as a fully integrated component, using a manufacturing method that is compatible with the process steps of a CMOS process as well as with so-called electroplating xe2x80x9cback endxe2x80x9d additive technology.
Thus, it is particularly advantageous that the sensitivity of the sensor element, and of the FET, can easily be adjusted with respect to the forces to be measured, i.e., accelerations, using the geometry of the springs, or spring structures, employed to achieve the substantially self-supporting suspension of the gate electrode. Furthermore, using the geometry of the springs and/or the spring structures, as well as using the thickness of the gate electrode and thus its inert mass, it is possible in a simple way to eliminate or minimize the transverse sensitivity of the FET, and of the sensor element, in a non-detection direction. In this way, it can be achieved that the gate electrode, under the influence of an external force, can be deflected only in one direction, which is oriented parallel to the surface of the substrate.
In particular, it is therefore possible, on the basis of the dimensions of the springs, or of the spring structures, and on the basis of the suspension of the gate electrode, to substantially avoid the width of the gap between the gate electrode and the channel region beneath changing under the influence of an impinging external force which especially has a component acting perpendicular to the surface of the substrate.
Furthermore, it is particularly advantageous if the gate electrode is supported by springs in a self-supporting manner such that in response to an external force, i.e., acceleration, acting upon the gate electrode, the overlapping surface of gate electrode and channel region, as seen in a top view of the gate electrode, changes at least approximately in a linear fashion with respect to a component of this external force acting parallel to the surface of the substrate. The characteristic curve of the FET, or sensor element, i.e., for example, the drain current as a function of the acceleration or of the external force, is therefore linear, given a suitable design of the gate electrode voltage applied, so that no additional circuit-engineering linearization is necessary.
Finally, the FET according to the present invention also makes it possible very advantageously to compensate for temperature fluctuations that arise. This is achieved, on the one hand, by a suitably designed spring structure, which connects the springs supporting the gate electrode in each case with their anchoring points, or, on the other hand, two FETs, arranged so as to be adjoining, are provided with a common gate electrode as the inert mass, the sum, in a top view, of the surface of the gate electrode overlapping the channel region of the first FET and the surface of the gate electrode overlapping the channel region of the second FET remaining at least approximately constant. In this manner, it is possible to suppress temperature fluctuations that may arise, or temperature dependencies of a sensor element of this type, effectively and without additional expense for circuitry.
Furthermore, it is also advantageous if, for increasing (stepping up) the measuring signal, i.e., the drain current for example, a plurality of FETs according to the present invention are connected in parallel.
Finally, it is advantageous that the FET according to the present invention is suitable not only for use in acceleration sensors, but also for use in other inertial sensors such as acceleration switches or rpm sensors or combinations thereof. Thus, the sensor element according to the present invention can be used, for example, also in restraint systems for motor vehicles, or in machine monitoring, it being marked by low manufacturing costs, high reliability, and long service life.